Season five is the best because it's Chandler and Monica trying to keep their relationship under wraps, which improbably stayed funny over the course of fourteen episodes and STILL managed to pay it off with the best episode of the series.

The writers didn't rush [the Monica and Chandler] relationship to its conclusion...and it's this comedy mined from their sneaking around and trying to keep their relationship a secret that makes Season 5 one of the show's best.